[1.01] Planetary Rings

P. D. Nicholson (Cornell)

A revolution in the studies in planetary rings studies
occurred in the period 1977--1981, with the serendipitous
discovery of the narrow, dark rings of Uranus, the first
Voyager images of the tenuous jovian ring system, and the
many spectacular images returned during the twin Voyager
flybys of Saturn. In subsequent years, ground-based stellar
occultations, HST observations, and the Voyager flybys of
Uranus (1986) and Neptune (1989), as well as a handful of
Galileo images, provided much additional information. Along
with the completely unsuspected wealth of detail these
observations revealed came an unwelcome problem: are the
rings ancient or are we privileged to live at a special time
in history? The answer to this still-vexing question may lie
in the complex gravitational interactions recent studies
have revealed between the rings and their retinues of
attendant satellites. Among the four known ring systems, we
see elegant examples of Lindblad and corotation resonances
(first invoked in the context of galactic disks),
electromagnetic resonances, spiral density waves and bending
waves, narrow ringlets which exhibit internal modes due to
collective instabilities, sharp-edged gaps maintained via
tidal torques from embedded moonlets, and tenuous dust belts
created by meteoroid impact onto parent bodies. Perhaps most
puzzling is Saturn's multi-stranded, clumpy F ring, which
continues to defy a simple explanation 20 years after it was
first glimpsed in grainy images taken by Pioneer 11. Voyager
and HST images reveal a complex, probably chaotic, dynamical
interaction between unseen parent bodies within this ring
and its two shepherd satellites, Pandora and Prometheus.

The work described here reflects contributions by Joe Burns,
Jeff Cuzzi, Luke Dones, Dick French, Peter Goldreich,
Colleen McGhee, Carolyn Porco, Mark Showalter, and Bruno
Sicardy, as well as those of the author. This research has
been supported by NASA's Planetary Geology and Geophysics
program and the Space Telescope Science Institute.